Apparatuses of the above-mentioned type are known in themselves and are described for example in U.S. Pat. Nos. 3,411,630, 3,501,011, 3,511,381, 3,516,548, 3,734,298, 3,837,496,, 4,051,041, 4,062,778 and 4,113,625, which all show apparatuses comprising a stack of spacer plates with membranes, preferably arranged in pairs between them.